Lead screw correcting device



Nov. 19, 1935. s. w. BATH El AL LEAD SCREW CORRECTING DEVICE s Sheets-Sheet 1 Filed July 13, 1934 S JL ' 176(12765071)". dzwrfey 21/. 7542/2. rwggi Wker fwliirfia M Nov. 19, 1935. s. w. BATH El AL 2,021 ,537

LEAD SCREW CORRECTING DEVICE Filed July 15, 19554 3 Sheets-Sheet 2 a vZ .5 @Z a2 0 f/weZA .Wlkefi 8 4d0lwey; {WY M NOV- 19, I S w BATH ETmAL LEAD SCREW CORRECTING DEVICE Filed July 15, 1934 I 3 Sheets-Sheet 3 Patented Nov. 19, 1935 UNITED STATES PATENT OFFICE LEAD SCREW' CORRECTING DEVICE Stanley W. Bath, Shrewsbury, and Ernest A. 7;. Walker, Worcester, Mass., assignors to John Bath & Company, Worcester, Mass, a corporation. of Massachusetts Application July 13, 1934, Serial No. 735,012

This invention relates to lathes, grinding machines and other machine toolsin which a lead screw is provided and in which extremelyaccurate feed must be maintained.

5 It has been found almost impossible, as well as prohibitive in cost, to produce lead screws for such machines which are absolutely uniform in pitch throughout their length.

It: is the object. of our invention to provide a correcting device by which a slightly inaccurate lead screw may be caused to produce the results which would otherwise require the use of a substantially perfect lead screw.

To the attainment of this object, we provide automatic means for shifting the lead screw bodily endwise in one direction or the other to counteract such slight inaccuracies as are found to exist in the pitch of the lead screw provided in a particular machine.

- Our invention further relates to arrangements and combinations of parts which will be hereinafter described and more particularly pointed out in the appended claim.

A preferred form of the invention is shown in the drawings, in which Fig. 1 is a front elevation, partly in section, of portions of a thread grinding machine having our improvements embodied therein;

Fig. 2 is an enlarged side elevation of certain parts, partly in section and taken along the line 22 in Fig. 1;

Fig. 3 is a plan view of a cam follower, looking in the direction of the arrow 3 in Fig. 2;

Fig. 4 is a partial plan view of a corrector bar, looking in the direction of the arrow 4 in Fig. 2;

Fig. 5 is an enlarged sectional front elevation, taken along the line 5-5 in Fig. 2;

Fig. 6 is a partial side elevation of an adjusting sleeve, and

Fig. 7 is a partial front elevation of a threaded bushing or nut.

Referring to the drawings, we have shown portions of a thread grinding machine including a base ID, a swivel table II, and a sliding table l2 mounted in a V-shaped guideway I3 of the swivel table I I and also having a portion engag-- ing a straight bearing surface l4 of said table II.

The work W may be supported on centers I6 and I1 and is engaged by a grinding wheel G mounted to rotate in a slightly oblique plane determined by the pitch of the thread to be ground.

The head center I6 is mounted in a work spindle 26 rotatable in bearings in a headstock 2| and driven from a motor M by any convenient driving connections, such as a pulley 22, belt 23,,

pulley 24, worm 25 and worm wheel 26, said worm wheel being on the work spindle 20.

A nut 30' is fixed on the upper side of the swivel table II and is threaded to receive a lead screw 3| which is connected by gears 32, 33, and 5 34 to the work spindle 20. The lead screw 3| (Fig. 5) is rotatable in an adjusting sleeve 40 (Fig. 5) and is provided with a flange 4| engaging one end of said sleeve. A collar 42 slidable but non-rotatable on the lead screw 3| is held in 10 close running engagement with the opposite end of the adjusting sleeve 40 by an adjusting nut 43.

The outer surface of the adjusting sleeve 40 is provided with a fine pitch thread, as indicated at 45 in Fig. 6, and is threaded into a nut or 15 threaded bushing 46, which is secured by screws 41 to a portion 48 of the sliding table l2.

A portion 49 of the bushing 46 is formed as a separate shoe, fitted in a slotted recess 50 (Fig.

7) and engaged by the end of an adjusting screw 20 5| (Fig. 2) threaded in the portion 48 of the sliding table I2. By adjusting the screw 5| all backlash between the adjusting sleeve 40 and the bushing 46 can be taken up.

An arm (Fig. 2) is secured by screws 6| to 25 the enlarged end portion of the adjusting sleeve 46. The arm 60 extends downward and is provided with a cam follower 62 having a reduced. end portion 63 adapted to engage a cam corrector bar 64. The bar 64 is secured by screws 30 65 to a bracket 66 (Fig. 1) extending outward from the base IU of the machine. A relatively heavy tension spring 68 (Fig. 2) holds the cam follower 62 in close engagement with the corrector bar 64.

The edge portion 10 (Fig. 4) of the corrector bar 64 which is engaged by the cam follower 63 is shaped to offset inaccuracies in pitch which are found by test to be present in the lead screw 3|.

By the use of precision blocks or other accurate instruments, the exact theoretical position of the table I 2 relative to the nut 36 after a certain number of revolutions of the lead screw 3| may be determined.

If the table is found to be slightly out of theoretical position at a given point, the edge 10 of the corrector bar 64 is built up or recessed a certain amount, thereby causing the arm 60 to 50 swing in one direction or the other as it is moved with the table along the corrector bar. Such swinging movement of the arm causes corresponding angular movement of the adjusting sleeve 40, which in turn effects a slight axial 55 bodily adjustment of the sleeve and lead screw relative to the sliding table [2.

If this correction is of the exact amount but in the opposite direction from the inaccuracy discovered by the use of instruments, the sliding table l2 will be brought to the correct theoretical position corresponding to the amount of rotation of the lead screw.

By taking instrumental readings at a series of points and by correspondingly building up or recessing the corrector bar 64 as indicated, the adjusting sleeve 40 may be caused to swing automatically in one direction or the other during the operation of the machine and thus ofiset inaccuracies along the entire length of the lead screw. Consequently, by the provision of a corrector bar 64, the lead screw 3| may be made theoretically correct in operation, any slight inaccuracies in the lead screw being offset by correcting movements of the sleeve 40, thus eliminating error.

While the production of an extremely accurate lead screw is extremely diflicult and very expensive, the production of a corrector bar such as 64 is a very simple and relatively inexpensive operation and renders a slightly imperfect lead screw entirely satisfactory in its operation.

Having thus described our invention and the advantages thereof, we do not wish to be limited to the details herein disclosed, otherwise than as set forth in the claim, but what we claim is:-

In a machine tool, a frame, a nut fixed on said frame, a sliding member, a lead screw extending parallel to the direction of sliding move- 5 ment of said member and threaded in said fixed nut, means to rotate said screw, an externally threaded adjusting sleeve in which said lead screw is rotatable but held from relative axial movement, an internally threaded device fixed on said 10 sliding member and supporting said adjusting sleeve and having threaded engagement therewith, the threads on said sleeve and in said device being of substantially finer pitch than said lead screw, an elongated corrector cam bar 15 mounted in fixed position on said machine tool and extending substantially parallel to said lead screw, a substantially elongated cam follower arm mounted on said adjusting sleeve and slidable along said cam bar, and means to hold the 2 end of said arm against said cam bar, said arm thereby adjusting said sleeve angularly in said fixed threaded device and correspondingly moving said lead screw bodily axially relative to said sliding member as said arm is moved along said 25 cam bar by said sliding member, whereby inaccuracies in pitch of said lead screw are corrected.

STANLEY W. BATH. ERNEST A. WALKER. 

